In recent years, charged-particle beam lithographic apparatus have attracted attention as tools for manufacturing LSI devices and VLSI devices. FIG. 9 schematically shows the simplest electron beam lithographic apparatus that is one example of a charged-particle beam lithographic apparatus. In this figure, an electron gun 101 emits an electron beam, which is focused by an electron lens 102 onto a material 103 to be patterned. The position of the focused spot is controlled by the deflecting force produced by a deflector 104. A signal indicating the focused spot positions is sent to the deflector 104 from a controller 105 via an amplifier 106. As a result, a pattern is written at desired positions on the material. After one field of pattern is written with the electron beam, a stage drive mechanism 109 moves a stage under the control of the controller 105 in such a way that the center of the next field of pattern to be written arrives at the electron optical axis.
Normally, the material 103 to be patterned is held on a cassette 107 which is carried on the stage 108 capable of moving in two dimensions, or along the X and Y axes. FIG. 6 shows one example of the cassette 107 used in an electron beam lithographic apparatus or the like to hold the material to be patterned. In FIG. 6, the body of the cassette is indicated by numeral 1. Upper surface-limiting plates 3 are mounted at three locations on the body 1 of the cassette to limit the position of the upper surface of a mask substrate 2 that is a material to be patterned. FIG. 7 is a cross section taken through one of the upper surface-limiting plates 3. A spring 4 is mounted on the opposite side of the substrate 2 from each upper surface-limiting plate 3. The spring 4 has a protrusion T at its front end. A spring holder 5 is provided with a hole 6 in which the spring 4 is held. The substrate 2 is inserted between the set of limiting plates 3 and the set of springs 4. Therefore, the substrate 2 is pressed against the limiting plates 3 by the springs 4. As a result, the upper surface of the substrate 2 is accurately flush with the lower surface of each upper surface-limiting plate 3. The X and Y coordinates of the substrate are limited by means of pins 7.
In placing the mask substrate 2 in position on the cassette body 1, the substrate 2 is locked and supported by the limiting plates 3 and the springs 4 at three positions located around the substrate 2. At these three positions, the upper surface of the substrate is accurately maintained at a given height by the upper surface-limiting plates 3. However, the substrate 2 yields under the weight of its own. The flexure was calculated in three dimensions. FIG. 8 shows the flexure of the substrate thus calculated. In FIG. 6, the dot-and-dash lines show contours of amounts of strains. The numerals in the circles represent the amounts of strains in .mu.m. The mask substrate 2 used for these calculations was a quartz glass plate 6 inch square and 0.09 inch thick. As can be seen from the contour map of FIG. 6, the strains produced on the mask substrate 2 do not have an orderly distribution because it is supported at three points.
Under this condition, if a pattern is written without correcting the beam shot positions, then the beam shot positions on the substrate differ from the desired ones. Consequently, the pattern is created with quite poor accuracy. Therefore, such errors between actual beam shot position and desired ones must be corrected. A countermeasure against this problem has been known, as disclosed in Japanese Patent Laid-Open No. 5517/1985. In particular, the amounts by which errors are corrected are measured at various locations on the material before a pattern is created on it. The measured amounts are stored in a memory. When a pattern is written at desired locations on the material in practice, appropriate amounts of correction are read from the memory. Then, the amount by which the electron beam is deflected or the amount of movement of the stage is adjusted according to the amounts of correction. Thus, the errors caused by the strains are corrected. Unfortunately, it is difficult to accurately forecast amounts of correction at locations other than the measured locations, because strains appear irregularly throughout the material to be patterned. For this reason, amounts of correction must be measured at numerous locations. Even if these numerous measurements are carried out in practice, it is impossible to correct the aforementioned errors accurately at other locations. Hence, the pattern is written with insufficient accuracy.